In conventional analog cellular communication systems, a plurality of base sites are used to provide associated geographical coverage areas within which radiotelephones communicate. Each base site is controlled by the system's central switch controller. When a radiotelephone call is to be handed-off from a communication channel at the host base site (the base site currently sustaining the call) to a communication channel at an adjacent base site, the system must determine which adjacent base site provides the best coverage area for the handoff. This is typically accomplished by instructing the equipment at each adjacent base site to monitor the channel used by the radiotelephone to be handed-off. Using scan receivers to monitor the radiotelephone channels, the equipment at each adjacent base site reports the signal strength of the radiotelephone's signal to the system's switch controller. The switch controller then compares the strongest reported signal strength with the signal strength from the radiotelephone at the host base site. When the latter falls below that of the former, the radiotelephone is instructed to reconfigure its transceiver to effect the handoff.
In a time division multiplex access (TDMA) cellular communication system, there are typically a plurality of radio frequencies employed at each base site to provide the appropriate allocation of voice communication channels for the base site's associated coverage area. Each frequency is time partitioned into several time slots, each of which may carry radiotelephone communication.
When a radiotelephone call is to be handed-off between coverage areas in a TDMA cellular communication system, the system also determines to which base site the call should be handed-off by the adjacent base sites monitoring the channel used by the radiotelephone. However, because each frequency may be carrying a radiotelephone communication, the base sites' scan receivers can become excessively busy monitoring each time slot in each frequency for each radiotelephone's signal strength.
This problem is aggravated when each base site includes only a few scan receivers which are time shared to monitor the frequencies. For each frequency on which at least one radiotelephone is a handoff candidate, the scan receiver must adjust its oscillator to that frequency and receive the information transmitted thereon. Such oscillator adjustment introduces a significant delay which is inherent in most every receiver. As the number of potential handoff candidates increases, this inherent delay can become unacceptable.
Accordingly, a scan receiver monitoring technique for such base sites in a TDMA cellular communication system is needed which overcomes the above described deficiencies.